The invention is based on a method for etching a silicon layered body as set forth by the species defined in the main claim.
The German laid open print 195 37 814 A1 describes a method for producing silicon layer systems, with which surface micromechanical sensors can be produced. To that end, deposited first on a silicon substrate is a thermal oxide upon which a thin layer of highly doped polysilicon is applied for use as a buried printed circuit trace. Deposited then on the polysilicon layer is a further oxide layer, and upon that, for example, a thick epipolysilicon layer. Finally, an aluminum metallization is deposited on the surface and patterned. The sensor structures to be exposed are subsequently etched out, preferably with a fluorine-based silicon deep-etching method described in the German patent 42 41 045. The sensor element is laid bare with the aid of a sacrificial-layer etching, in which the oxide below the sensor regions is removed by media containing hydrofluoric acid using a vapor-etching method. Disadvantages in this undercutting technique is that the oxide is removed not only under the sensor region to be exposed, but also above and partially even under the polysilicon printed circuit traces, so that there is the danger of shunts and leakage current. Protection of the oxide regions whose undercutting is to be prevented, for instance, by protective lacquers, is only possible with considerable expenditure, since vaporous hydrofluoric acid penetrates almost all practicable polymer protective layers very quickly, and moreover, can act very
The German patent 44 20 962 A1 describes a dry-etching method in silicon for producing sensor structures by a combination of anisotropic and isotropic plasma-etching techniques. A subsequent wet-etching step or etching in the vapor phase is not necessary in this case. All the process steps can be carried out in one single plasma-etching installation. To that end, first of all, the sensor structure is produced with vertical walls with the aid of the anisotropic deep-etching method described in DE 42 41 045. In this context, deposition steps in which a Teflon-like polymer is deposited on the side wall, and fluorine-based etching steps which are isotropic as such and are made locally anisotropic by propelling the side-wall polymer during the etching, alternate. The silicon substrate is subsequently isotropically etched with a fluorine-based etching step until the silicon structure for the sensor element is completely exposed. However, this method has two serious disadvantages. On one hand, because of the “microloading effect”, narrow etched trenches are etched more slowly than wide etched trenches, which then also holds true for the speed of the subsequent lateral undercutting, i.e., the undercutting progresses more slowly for narrow trenches than for wide trenches. Secondly, the structures to be exposed are also attached from their lower side or bottom. The result is that structures which are surrounded by wide trenches have less residual height than structures which are surrounded by narrow trenches, which frequently leads to unreproducible and unsatisfactory mechanical properties of the sensor elements produced.